Electrochemical sensor for the specific detection of peroxyacetic acid in aqueous solutions using pulse amperometric methods

ABSTRACT

An electrochemical sensor (A, A′) is specific for the detection of peroxyacetic acid in a solution which also contains hydrogen peroxide. A potential is applied between a reference electrode ( 120, 120′ ) and a working electrode ( 118, 118′ ). A read voltage (FIG.  7 ) is selectively pulsed across a counter electrode ( 122, 122′ ) and the working electrode. The current flowing between the working electrode and the counter electrode is dependent on the peroxyacetic acid concentration in the solution (FIG.  6 ). By careful selection of the read voltage, the contribution of hydrogen peroxide to the current flow is virtually negligible. The sensor effectively measures peroxyacetic acid concentrations in the range generally employed in sterilization and disinfection baths (100-3000 ppm.).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to the sterilization anddisinfection arts. It finds particular application in conjunction withthe detection of peroxyacetic acid concentrations in solutions used forsterilization or disinfection of medical, dental, and pharmaceuticalequipment and will be described with particular reference thereto. Itshould be appreciated, however, that the invention is also applicable todetection of peroxyacetic acid and other oxidizable chemicals insolution, such as hydrogen peroxide.

[0002] Peroxyacetic acid, or peracetic acid, is a useful sterilantand/or disinfectant for a variety of applications, includingdisinfection of waste and sterilization or disinfection of medical,dental, pharmaceutical, or mortuary equipment, packaging containers,food processing equipment, and the like. It has a broad spectrum ofactivity against microorganisms, and is effective even at lowtemperatures. It poses few disposal problems because it decomposes tocompounds which are readily degraded in sewage treatment plants.Peroxyacetic acid solutions also have the ability to be reused over aperiod of time, allowing instruments to be sterilized or disinfectedthroughout the day in the same bath of sterilant.

[0003] In use, peroxyacetic acid precursors are typically mixed withwater and other chemicals in order to create a sterilant solution. Itemsto be sterilized or disinfected are then immersed in the sterilant.Decontaminated items are then rinsed to remove traces of the acid andother cleaning chemicals, before use. To ensure effective sterilizationor disinfection within a preselected period of time, the concentrationof peroxyacetic acid is maintained above a selected minimum effectivelevel. Disinfection is typically carried out at lower concentrations ofperoxyacetic acid than for sterilization. When the peroxyacetic acidconcentration is at or above the minimum effective level, completesterilization or disinfection is expected.

[0004] Because the peroxyacetic acid tends to decompose over time, it isvaluable to monitor the sterilant periodically to determine the level ofperoxyacetic acid. The level can be compared against preselected minimumlevels, used to adjust contact time, used to control concentration, orthe like. Currently, it is often assumed that the sterilant will remainat or above the minimum effective concentration. However, differences inthe temperature of the sterilant, the quantity of items sterilized ordisinfected, and the degree and nature of contamination of the items allresult in considerable variations in the degradation of the sterilant.In addition, storage conditions and duration sometimes lead todegradation of the peroxyacetic acid precursors before use.

[0005] Methods currently used to detect peroxyacetic acid are oftenunable to distinguish between peroxyacetic acid and other compoundstypically present in the solution, such as hydrogen peroxide and aceticacid. Dippable papers are easy to use, but lack accuracy, particularlyat concentrations suitable for sterilization or disinfection. Chemicaltitration methods provide a more accurate measure of the peroxyaceticacid in solution, but are time consuming to perform and are not readilyautomated. Frequently, more than one titration is performed to eliminatethe contribution of hydrogen peroxide to the result.

[0006] Recently, a number of electrochemical techniques have beendeveloped for detection of oxidizable or reducible chemical species,such as peroxyacetic acid. Consentino, et al., U.S. Pat. No. 5,400,818,discloses a sensor for peroxyacetic acid-hydrogen peroxide solutions.The sensor measures the resistivity of the solution, which is dependenton both the peroxyacetic acid and the hydrogen peroxide concentrations,as well as other factors. Thus, the sensor is unable to differentiatebetween the two components.

[0007] European Patent Application EP 0 333 246 A, to Unilever PLC,discloses an electrochemical sensor for detection of oxidizable orreducible chemical species using an amperometric method in which a fixedpotential is maintained between a reference and a working electrode. Thecurrent at the working electrode is used to determine the concentrationof peroxyacetic acid. Other species present, however, influence thecurrent flowing, and hence the accuracy of the results.

[0008] Teske, U.S. Pat. No. 5,503,720, discloses a process for thedetermination of reducible or oxidizable substances, such asperoxyacetic acid in sewage waste. The process uses potentiostaticamperometry to detect peroxyacetic acid concentrations. The technique,however, depends on the achievement of a steady state, which frequentlytakes several hours.

[0009] Conventional electrochemical detection systems often employ aporous membrane, which separates the sample to be analyzed from theelectrodes. Charged species pass through the membrane when traveling tothe electrodes. This increases the time for measurements to be made andadds complexity and cost to the system.

[0010] The present invention provides a new and improved sensor andmethod for the selective detection of peroxyacetic acid which overcomesthe above referenced problems and others.

SUMMARY OF THE INVENTION

[0011] In accordance with one aspect of the present invention, adecontamination process is provided. The process includes circulating adecontaminant solution including peroxyacetic acid though a treatmentvessel which contains items to be decontaminated. The process furtherincludes withdrawing a sample of the decontaminant solution into achamber to contact a working electrode and a counter electrode andpulsing a voltage between the working electrode and the counterelectrode at a selected voltage relative to a reference electrode andmeasuring the output current generated. The voltage is selected suchthat the current generated is substantially dependent on a concentrationof the peroxyacetic acid in the sample and substantially independent ofa concentration of another oxidizing species in the sample.

[0012] In accordance with another aspect of the present invention, amethod of detecting a first oxidizing species in a solution to be testedis provided. The solution also contains a second oxidizing species. Themethod includes disposing a working electrode and a counter electrode inthe solution, pulsing a read voltage in the diffusion limiting rangeacross the working electrode and the counter electrode, and detectingcurrent flowing between the working electrode and the counter electrode.The read voltage is selected such that the current flowing issubstantially dependent on the concentration of the first oxidizingspecies and substantially independent of the concentration of the secondoxidizing species in the solution.

[0013] In accordance with another aspect of the present invention, adecontamination apparatus is provided. The apparatus includes adecontamination vessel which receives items to be decontaminated. Afluid flow path circulates a decontaminant ion solution through thevessel. A sensor system is fluidly connected with the fluid flow pathfor specifically detecting the decontaminant in the decontaminantsolution. The system includes a chamber which receives a sample of thedecontaminant solution from the fluid flow path, a working electrode,and a counter electrode disposed within the chamber to contact thesample of decontaminant solution. An amperometric controller iselectrically connected with the working and counter electrodes. Thecontroller selectively pulses a preselected read voltage between theworking electrode and the counter electrode and detects an outputcurrent flowing in a circuit including the working electrode, thecounter electrode, and the solution. The read voltage is selected suchthat the output current is substantially dependent on the decontaminantconcentration and substantially independent of the concentration ofanother oxidizing species in the solution.

[0014] One advantage of the present invention is that it enables theperoxyacetic acid concentration of a sterilizing or disinfectingsolution to be determined rapidly, (i.e., in less than one minute) andwithout interference by other oxidizing species present in the solution.

[0015] Another advantage of the present invention is that the sensorconfirms that a minimum effective concentration of peroxyacetic acid ismaintained for effective sterilization or disinfection.

[0016] Yet another advantage of the present invention is the provisionof a disposable sensor probe that requires no calibration before use.

[0017] Still further advantages of the present invention will becomeapparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention may take form in various components andarrangements of components, and in various steps and arrangements ofsteps. The drawings are only for purposes of illustrating a preferredembodiment and are not to be construed as limiting the invention.

[0019]FIG. 1 is a plumbing diagram of a peroxyacetic acid sterilizationand disinfection system according to the present invention;

[0020]FIG. 2 is a schematic diagram of an electrochemical system fordetecting peroxyacetic acid using pulse amperometry according to thepresent invention;

[0021]FIG. 3 is a top view of a disposable sensor for detectingperoxyacetic acid according to one embodiment of the present invention;

[0022]FIG. 4 is a side view of a reusable sensor system, according toanother embodiment of the present invention;

[0023]FIG. 5 is a side view through section V-V of the reusable sensorsystem of FIG. 4;

[0024]FIG. 6 is a plot showing a pulse sequence applied between workingand counter electrodes of the sensor system of FIG. 1; and

[0025]FIG. 7 is a simulated plot of current flow against read voltagefor solutions containing peroxyacetic acid and hydrogen peroxide.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] With reference to FIG. 1, a peroxyacetic acid monitoring systemor sensor A measures peroxyacetic acid concentrations in the presence ofhydrogen peroxide. The monitoring system will be described withreference to an automated liquid decontamination apparatus 1 whichsequentially cleans items, such as endoscopes or other medical, dental,and pharmaceutical devices, and the like, and then sanitizes,sterilizes, or disinfects them with a decontaminant solution whichcontains peroxyacetic acid. It should be appreciated, however that themonitoring system is also applicable to the measurement of peroxyaceticacid concentrations in other treatment systems and peroxyaceticacid-containing liquids.

[0027] The term “decontamination” and other terms relating todecontaminating will be used herein to describe sanitizing,sterilization, disinfection, and other antimicrobial treatments whichare designed to destroy microorganisms contaminating the items.

[0028] The system 1 includes a decontamination cabinet 10 which definesan interior decontamination chamber 12. Items to be sterilized,disinfected, sanitized, or otherwise microbially decontaminated areloaded into the decontamination chamber through an opening in a frontwall 13 of the cabinet, illustrated as closed by a door 14. Within thechamber, several spray jets or nozzles 16 spray a decontaminant solutionover the items. Optionally, in the case of instruments with lumens, orother internal passages, some of the nozzles act as fluid ports 18 whichare configured for interconnection with internal passages of theendoscopes and other objects with lumens, for supplying decontaminantsolution and other liquids to the internal passages.

[0029] A collection tank or sump 20 forms the base of the cabinet 10 andreceives the sprayed decontaminant solution as it drips off the items. Ahigh pressure pump 22 delivers the decontaminant solution under pressureto the nozzles 16 and fluid ports 18 through a fluid distribution systemor manifold 24.

[0030] A source 30 of a decontaminant solution preferably includes awell or mixing chamber 34. The well receives a dose of a concentrateddecontaminant, such as an antimicrobial agent or reagents which react toform an antimicrobial agent on mixing with water. As shown in FIG. 1,the well is preferably integral with the collection tank 20 of thechamber, although a separate well is also contemplated.

[0031] A preferred antimicrobial agent is peroxyacetic acid, either inconcentrated liquid form, or as a reaction product of powdered reagents,such as acetyl salicylic acid and sodium perborate. A water inlet 42supplies water, typically from a municipal water system to the well. Thewater mixes with detergents, corrosion inhibitors, the concentrateddecontaminant, and other selected components in the well to form wash,decontaminant, or other solutions.

[0032] Preferably, the concentrated decontaminant and the othercomponents are supplied in a disposable package or cup 44 which ispositioned in the well 34 prior to a decontamination cycle. The cup 44holds a measured dose of the concentrated decontaminant. Optionally, acleaner concentrate is also contained in the cup for forming a cleaningsolution to clean the items prior to antimicrobial decontamination. Thecup 44 may include a number of compartments which separately contain thecleaning concentrate and decontaminant concentrate for separate releaseinto the system. In this way, the items are first cleaned and thenmicrobially decontaminated.

[0033] In a preferred embodiment, the cup holds a cleaning concentratein a first compartment. A second compartment holds pretreatmentcomponents, such as buffers for adjusting the pH, surfactants, chelatingagents, and corrosion inhibitors for protecting the components of thesystem and items to be decontaminated from corrosion by thedecontaminant. A decontaminant, such as concentrated liquid peroxyaceticacid solution (or reagents that react to form it) is held in a thirdcompartment. A cup cutter 46, or other suitable opening member, ispositioned at the base of the well 34 for opening selected compartmentsof the cup, in sequence.

[0034] Alternatively, a solid or liquid concentrated decontaminant issupplied to the system from a separate bulk source (not shown), or issupplied to the system as the decontaminant solution, in analready-diluted form.

[0035] The water used for diluting the cleaner concentrate anddecontaminant may be tap water or treated water, such as distilledwater, filtered water, microbe free water, or the like. The quantity ofwater entering the system is regulated to provide a decontaminantsolution of a desired concentration in the decontamination chamber 12.The water is preferably passed through a microporous filter 50 in thewater inlet line 42 which filters out particles of dirt andmicroorganisms. A valve 52 in the water inlet 42 closes when theselected quantity of water has been admitted.

[0036] A fluid supply pathway 60 connects the well 40, he pump 22, andthe fluid distribution system 24. A heater 64, situated in the fluidsupply pathway 60, heats the decontaminant solution and optionally thecleaning solution and rinse liquid to a preferred temperature(s) foreffective cleaning, decontamination, and rinsing. A temperature of about50-60° C. is preferred for sterilization with peroxyacetic acid. Thepathway 60 returns the sprayed decontaminant solution from the sump 20to the manifold 24, and thence to the nozzles 16 and the fluid ports 18via a recirculation valve 68. At least a portion of the sprayeddecontaminant solution is directed through the well 34 before beingreturned to the decontamination chamber. This ensures thorough mixing ofthe concentrated decontaminant and other components with the solutionbefore returning the decontaminant solution to the nozzles 16, 18.

[0037] The peroxyacetic acid monitoring system A detects theconcentration of peroxyacetic acid passing through the fluid lines. FIG.1 shows the system connected with the line 60. It should be appreciated,however, that the sensor is also conveniently connected with or disposedin any of the fluid flow lines of the system. A computer control system80 controls the operation of the peroxyacetic acid monitoring system A.Preferably, the control system also controls the operation of otherelements of the system 1, including the introduction of the cleanerconcentrate, the peroxy concentrate, and other reagents as well as thepump 22, the heater 64, the valves 52, 68 and the like. The controlsystem 80 may control one or more additional systems 1, if desired.

[0038] With reference now to FIG. 2, the system A for selectivedetection of peroxyacetic acid includes an electrode system 110, and anamperometric controller 112. The controller 112 both applies voltagesand detects current flows in the system A. Although the controller isshown as a single unit in FIG. 2, it should be understood that acombination of pieces of electrochemical equipment generally known inthe art which serves these functions is also contemplated.

[0039] The electrode system 110 is disposed in a reservoir 114 whichreceives a peroxyacetic acid solution to be tested, or may be placeddirectly in the recirculation path of the automated processor 1.Preferably, as shown in FIG. 1, the reservoir comprises a separatechamber, into which a sample of the circulating decontaminant solutionis withdrawn at intervals. Because peroxyacetic acid is generally inequilibrium with hydrogen peroxide when in solution, the solution to betested invariably contains some hydrogen peroxide.

[0040] The electrode system 110 includes three electrodes, namely aworking electrode 118, a reference electrode 120, and a counterelectrode 122. The reference electrode produces a constant electricalpotential (or base potential). A suitable reference electrode 120 is asilver/silver chloride electrode. The working electrode is preferably anelectroactive for peroxyacetic acid, such as carbon, gold (either alone,or doped with an inert material), or platinum. A particularly preferredworking electrode 118 is amorphous, glassy carbon. Carbon is aneffective electroactive for peroxyacetic acid and is highly selectivefor peroxyacetic acid in the presence of hydrogen peroxide. Carbonelectrodes are also relatively resistant to peracetic acid, giving thema longer useful life. Glassy carbon is a particularly effective workingelectrode for measurements in the diffusion limited region. The counterelectrode 122 is preferably formed from an inert conductive material,such as carbon, which readily accepts electrons. Alternatively, suitablecounter electrodes are formed from silver, gold, or titanium.

[0041] With reference to FIG. 3, in one embodiment, a substrate 124,preferably formed from an inert polymeric or ceramic sheet, supports theelectrode system 110 to form a disposable probe 125. Electric leads 126electrically connect the electrodes and the controller 112 throughconnecting points 128. Optionally, the sensor probe also includes aninsulation layer 130 which covers the substrate and the leads around theconnection points. The insulation layer inhibits the leads fromparticipating in the electrochemical reactions. Optionally, a thermistor132 detects the temperature of the sample in the region around theprobe.

[0042] The sensor probe 125 is preferably constructed by thin filmprinting technology, although other methods of probe formation are alsocontemplated. In one embodiment, components of the sensor, includingelectrodes, electrical connection points and electrical leads are alllaid down on the substrate. Materials for the electrodes and connectionpoints are separately dispersed in inks and printed onto the substrate.The inks are cured, for example, by heat, UV light, or the like. Theprobes produced are inexpensive and thus are suited to single use.Additionally, such probes can be used without prior calibration. Theelectrode materials are selected so that they will not become disbandedwhen immersed in a peroxyacetic acid solution at temperatures betweenaround 25° C. and 75° C. The choice of ink also affects the conductivityto some degree.

[0043] In another embodiment, shown in FIGS. 4 and 5, where likecomponents are numbered with a prime (′), a sensor A′ includes adurable, reusable electrode system 110′ is shown. The electrode system110′ comprises a working electrode 118′, a reference electrode 120′ anda counter electrode 122′. The electrodes are analogous to thosedescribed above for the disposable sensor, but in this embodiment, areconstructed to be reusable. The electrodes are mounted in a housing 150formed from stainless steel or other material with a large heatcapacity.

[0044] The housing 150 defines an interior chamber or reservoir 114′.Working faces of the three electrodes 118′, 120′, 122′ project throughwalls 156 of the housing into the chamber. The electrodes are sheathedwith and receive mechanical support from insulating material 158 so thatonly the working faces are exposed to the peroxyacetic acid sample.Steel tubes 160 are threadably, or otherwise removably attached to thewalls of the chamber and carry the electrodes therethrough for ease ofinsertion and removal of the electrodes from the chamber and formechanical support exterior to the chamber.

[0045] An inlet line 162 carries a sample of the circulatingdecontaminant solution into the chamber through an inlet 164 formed inone wall of the housing 150. A diaphragm valve 168 in the inlet line isnormally closed, except when a sample is being taken. An overflow ordrain line 170 carries fluid from the chamber via an outlet 172 definedthrough an opposite wall of the chamber. The overflow line leads to adrain via an inverted U-bend or trap 174 or returns the sample to thefluid flow line 60. It is preferred to direct the decontaminant solutionto drain since this eliminates the need to assure sterility of reservoirsurfaces of the sensor housing.

[0046] The chamber 114′ and housing 150 are configured such that thethermal mass of the housing is substantially greater than the volume ofthe decontaminant solution to be sampled. The internal volume of thechamber is preferably about 10-15 ml or less. One or more thermalelements 176, within the walls 156 of the housing, maintains the housingat a stable temperature, and thereby the sampled fluid. Preferably, thesample is heated to a measurement temperature only slightly above themaximum temperature expected in the circulating fluid. This allows thesample to reach the measurement temperature very quickly. For example,if the decontamination portion of the cycle operates at about 50-55° C.,the walls are preferably maintained at about 60° C. Alternatively, thesample may be cooled by cooling elements, such as by Peltier elements,to achieve an optimum measuring temperature. A thermocouple 132′, orother temperature detector, detects the temperature of the chamber wallsor the sampled fluid in the chamber. A temperature detector 178 receivessignals from the thermocouple and adjusts the thermal elements tomaintain the walls at a constant temperature. Alternately, compensationfor temperature fluctuations can be made in the calculation ofconcentration, the currents from the electrodes, or the like.Preferably, the large, heated mass of the housing quickly brings thesample to a reproducible as well as stable temperature.

[0047] When a sample is to be taken, the valve 168 opens and allows thesampled fluid to flow into the chamber. The valve 168 remains open forsufficient time to allow the sampled fluid to flush the contents of thechamber through the overflow and replace the contents with freshlysampled fluid. In the system 1, the pump 22 pressurizes the circulatingdecontaminant to about 70 psi. In this case, a flush and fill period ofaround three seconds is sufficient to fill the chamber with a freshsample of decontaminant solution. The valve is then closed and thesample is held within the chamber for sufficient time to equilibrate thetemperature and for the sampled fluid to become quiescent. Once thisequilibration period is complete, a pulsed voltage sequence is appliedto the electrodes, resulting in the generation of an electrical currentwhich is correlated to the concentration of peroxyacetic acid in thesample. The sampling and measurement steps are repeated, preferablyevery one to two minutes, to ensure that the peroxyacetic acidconcentration does not drop below a minimum acceptable level.

[0048] With reference once more to FIG. 2, the amperometric controller112 includes a voltage regulator 180 which applies a reference voltage(relative to the potential generated by the reference electrode) betweenthe reference electrode 120 and the working electrode 118 of theembodiment of either FIG. 3 or FIGS. 4 and 5. A voltage pulser 182superimposes a read voltage between the working and counter electrodesin short pulses.

[0049] Since reference electrodes do not tend to conduct electricitywell, this may lead to resistance problems. It is desirable for thecounter electrode 122 to be held at a potential sufficient to preventcurrent from flowing through the reference elecrode. This is readilyachieved by using operational amplifiers 184, connected between thereference electrode 120 and counter electrode 122, which hold thereference electrode and the counter electrode at the same electricalpotential. The amplifiers only allow current to flow through the sampledsolution between the working and the counter electrode. This allowsprecise control of the applied potential while blocking the referenceelectrode against carrying electrical current. The reference potentialof the reference electrode is thus used to calibrate/control the voltagepotential applied between the working and counter electrodes so that thesignal generated is well controlled.

[0050] The controller 112 also includes a current monitor 186 whichdetects the current flowing between the working and counter electrodes.

[0051] At a given temperature, the current measured is dependent on boththe peroxyacetic acid concentration and the concentration of otheroxidizing species, such as hydrogen peroxide, in the solution tested.The respective contributions of each of these components to the overallcurrent measured is dependent on the selected read voltage. Over alimited read voltage range, which is partially dependent on thetemperature of the solution, the hydrogen peroxide (or other oxidizingspecies present) has a much smaller influence on the current than theperoxyacetic acid. Thus, by carefully selecting a read voltage whichminimizes the effect of other oxidizing species, the current measured isvirtually independent of the concentration of hydrogen peroxide andshows a linear relationship with peroxyacetic acid concentration. Forsolutions containing peroxyacetic acid and hydrogen peroxide, the readvoltage is preferably in the range of −1.2 volts to −1.6 volts, morepreferably about −1.4 volts, relative to an Ag/AgCl reference electrode,when the working electrode and counter electrode are both carbon. Whenthe working and counter electrode is gold, a preferred read voltage isabout −50 mV. Obviously, if a different reference electrode andcorresponding different base potential are employed, the read voltagerelative to the reference electrode will change accordingly.

[0052] With reference now to FIG. 6, a typical measurement sequenceincludes a preconditioning phase P and a read phase R. Thepreconditioning phase enhances the quality of the current signalreceived in the read phase. A preferred preconditioning phase P includesapplying a conditioning voltage pulse between the working electrode 118and counter electrode 122 at about −2 volts for about 2 seconds,followed by about +2.5 volts for about 4 seconds. A sensing pulse at theread voltage (about −1.4 volts) is then applied for about 10 seconds.

[0053] The current flowing during the read pulse decays asymptoticallyto a steady value. Preferably, the current is measured towards the endof the application of the read voltage when its value has substantiallyreached steady state. For example, the current flowing during the lasttwo to three seconds of the sensing pulse (the read phase) is measuredand averaged to produce the signal that is used to measure theconcentration of peroxyacetic acid. This allows time for the decay ofany double layer formed on the measurement electrode and theestablishment of a diffusion-limited current so that the currentmeasured is derived from primarily faradaic reactions, rather than theprimarily capacitative currents which occur in the double layer.

[0054] The pulse sequence of FIG. 6 is repeated a plurality of timesduring the antimicrobial stage of the cycle, each time with a new sampleof the circulated solution. Any residue build-up at the end of cycle iselectrochemically removed at the beginning of the next cycle. Morespecifically, after the last measurement, liquid is retained in theelectrolytic cell either by retaining the last sample or by filling thecell with rinse water in the subsequent rinse stage.

[0055] In the next cycle, the cell samples the solution after thebuffers, wetting agents, and corrosion inhibitors have been circulated,but before the antimicrobial is added. With the sampled buffer solution,voltage pulses are applied between the read and counter electrodes todrive of f the residue at a voltage about the voltage that causeshydrogen gas to form and below the voltage at which oxygen gas forms onthe working electrode of the present configuration. The pulses are largeenough to drive off the residue, but small enough that the carbonelectrode is not electrochemically eroded. In the present system,alternating square wave pulses of −2.0 volts for 2 seconds and +2.5volts for 4 seconds are preferred. However, voltage pulses of −1.5 to−2.5 volt and +2.0 to +3.5 volts with durations of 1-10 seconds can alsoproduce satisfactory results.

[0056]FIG. 7 illustrates the relationship between concentration andcurrent flow for solutions containing fixed concentrations of hydrogenperoxide and peroxyacetic acid. At the read voltage (around −1.4v forthe system described above, relative to silver/silver chloride), theperoxyacetic acid makes a much larger contribution to the currentmeasured than the hydrogen peroxide present. Electrode systems 110 andapplied voltages are readily designed which allow the contribution ofperoxyacetic acid to the current measured to be roughly ten times thatof hydrogen peroxide, or greater. Unless the concentration of hydrogenperoxide in the solution to be tested is much greater than that ofperoxyacetic acid, the current output in the optimal read voltage rangeis, for all practical purposes, dependent on the peroxyacetic acidconcentration.

[0057] In some instances the optimum read voltage may not be achievablein the electrochemical system due to background noise. At low voltages(about −20 mV), the current output tends to be masked by backgroundnoise and therefore measurement of very low peroxyacetic acidconcentrations, in particular, may be difficult. Thus, the choice of apreferred read voltage is dependent on the likely concentrations ofperoxyacetic acid to be measured, the ratio of peroxyacetic acid tohydrogen peroxide, and the degree of background noise in the system.Using a carbon electrode pushes the optimum read voltage away from thebackground noise region (−20 mV). Read voltage pulses at about −1.4volts relative to silver/silver chloride are ideal for detectingperoxyacetic acid concentrations in the range of 100 ppm to 3000 ppm,when the hydrogen peroxide concentration is less than, or notsubstantially greater than, the peroxyacetic acid concentration.

[0058] The choice of materials for the electrodes thus affects theselection of a preferred read voltage. Other factors, such astemperature and pH, also influence the selection.

[0059] The current output increases with temperature. Preferably, theheated housing brings the temperature of the sample to a constanttemperature for measurements to be made.

[0060] Alternatively, where significant variations in temperature areanticipated, the detected current flows are preferably corrected forvariations in the temperature. The thermistor 132, in this embodiment,is placed in contact with the sample to be tested and measures thetemperature of the peroxyacetic acid sample. Current measurements arethen compensated for variations in temperature. The computer controlsystem 80 optionally corrects the detected current flows for variationsin temperature detected by the thermistor 132. The computer preferablyaccesses a look-up table 196 and determines the peroxyacetic acidconcentration corresponding to the current output measured. However, forautomated processing systems where temperatures are controlled to within±2-3° C., the effect of temperature on the current is relatively smalland thus temperature compensation may be unnecessary.

[0061] The current generated is dependent upon the surface area of theworking 118 and the counter electrode 122. Preferably, the workingelectrode surface area is significantly smaller than that of the counterelectrode. Thus, the current flow generated for a given peroxyaceticacid concentration is limited by the working electrode surface area. Thecounter electrode has a larger surface area than the working electrodeto avoid saturation of the electrode with electrons and a loss of thelinear relationship between peroxyacetic acid concentration and currentflow at higher peroxyacetic acid concentrations.

[0062] The read voltage is pulsed between the working and counterelectrodes at a fixed rate. A preferred rate of pulsing is around 25 Hz(samples/sec). Because of rate limiting diffusion effects, the currentoutput decreases asymptotically with time, eventually reaching a plateauregion in which the current output is relatively constant with time. Asample time of around 5-15 seconds allows such steady state conditionsto be established. The controller 112 then detects the current output,from which the peroxyacetic acid concentration is determined. Betweeneach sampling period, the working electrode returns to the referencevoltage for a period of around 5 seconds.

[0063] By repeating the sampling and the measurement of current outputover a period of time, at intervals of about 30 seconds to two minutes,an accurate current measurement of the peroxyacetic acid concentrationin the sterilant solution or in the sample is obtained.

[0064] When peroxyacetic acid is present in the sample, the workingelectrode becomes enriched with electrons when the sensing pulse isapplied. This excess of electrons will tend to cause the peroxyaceticacid molecules in the vicinity of the electrode to become reduced (i.e.,accept electrons from the surface of the electrode.) The movement ofelectrons from the electrode into the solution via this mechanismproduces the electrical current that can be measured.

[0065] The magnitude of the current produced is proportional to theconcentration of the peroxyacetic acid molecules close to the surface ofthe electrode. When the magnitude of the voltage is small, the rate atwhich the peroxyacetic acid molecules react is slow compared to the rateat which the peroxyacetic acid at the surface is replenished bydiffusion from the bulk solution. As the voltage is increased, the rateat which peroxyacetic acid is consumed increases and, as timeprogresses, the concentration of peroxyacetic acid close to theelectrode is depleted. This results in the current droppingexponentially and asymptotically reaching a limit determined by the rateat which peroxyacetic acid can diffuse from the bulk solution to thesurface of the electrode (i.e., a diffusion limited current). Theperoxyacetic acid sensor A, as it is used herein, measures thisdiffusion limited current. The decontaminant solution contains abuffering system which acts as an electrolyte. When a voltage isapplied, a small current will flow due to the electrical conductivity ofthe electrolyte. In addition, when chemical species are present that aresusceptible to electrochemical conversion, an additional electricalcurrent will be produced due to electrochemical conversions at thesurfaces of the electrodes.

[0066] In one embodiment, the computer control system 80 signals analarm 202 when the peroxyacetic acid concentration of the bath dropsbelow a preselected minimum peroxyacetic acid concentration. Or, thecomputer adjusts the length of the cycle to compensate for a reducedperoxyacetic acid concentration.

[0067] In another embodiment, the control system 80 adjusts theconcentration of peroxyacetic acid flowing through the system inresponse to the detected concentration. In this embodiment, the controlsystem signals a valve 204 in fluid communication with the fluid line 60to open and release an additional dose of the concentrated source ofperoxyacetic acid into the system from a supplementary dispenser, suchas a reservoir 206, or other source of the concentrate. Other means ofadjusting the peroxyacetic acid concentration are also contemplated.

[0068] Because the electrodes 118′, 120′, 122′ in the reusable sensor A′tend to degrade over time, they should be replaced at intervals tomaintain the accuracy of the sensor. Optionally, a calibration check iscarried out prior to a sterilization cycle with a peroxyacetic acidcontaining solution or solutions of known concentration, preferablyconcentrations in the range to be measured. The reference electrode 120′may be checked every decontaminant cycle by measuring the magnitude ofthe reference potential relative to the carbon electrodes 118′, 120′and/or the stainless steel housing in the presence of an electrolyte.The electrolyte may be the pretreatment mixture of buffers, corrosioninhibitors, and the like, which is circulated through the system priorto addition of the peroxyacetic acid decontaminant.

[0069] It is also important to maintain the surface condition of theworking electrode, since the specific area of the working electrodeaffects the magnitude of the measured current. Conductivity measurementmay be made periodically to provide information on the state of theelectrode surface. For example, the conductivity is measured each cyclein the presence of the buffers and corrosion inhibitors. Provided thatthe ionic strength of the buffered solution does not vary significantlyfrom cycle to cycle, the conductivity measurements can be used toprovide an indication of the state of the working electrode surface.Theoretically, the electrical resistance between the housing and theworking electrode is a function on both the surface area of the housingand the surface area of the working electrode. Since the surface area ofthe housing is significantly larger than that of the working electrode,the electrical resistance will be more sensitive to changes in surfacearea of the working electrode.

[0070] When the electrodes 118′, 120′, 122′ are to be reused, it ispreferable to maintain the working surfaces in contact with anelectrolyte or water between decontaminant cycles. Accordingly, a sampleof the decontaminant solution is left in the chamber at the end of acycle. Or, the chamber is filled with a fresh solution of electrolyte orrinse water. Particularly when the system 1 is not to be used for sometime, the electrodes may be removed from the sensor A′ and stored in anelectrolyte solution or water until needed.

[0071] In an alternative embodiment, one or more of the electrodes isdisposable, while the remaining are reusable. For example, a card typesensor 125 of the type shown in FIG. 3 may be used for the working andcounter electrodes 118, 122 in combination with a reusable referenceelectrode 120′ of the type shown in FIGS. 4 and 5. The card is disposedafter a decontamination cycle, and the reference electrode 120′ isreused.

[0072] It will be appreciated that the peroxyacetic acid monitoringsystem A, A′ may also be used in a variety of other peroxyacetic acidsterilization/disinfection systems in which items to be microbiallydecontaminated are immersed in, or sprayed with a peroxyacetic acidsolution. The system may also be used to detect the concentration ofperoxyacetic acid in a bath containing peroxyacetic acid or in fluidflow lines of a water treatment system, bleaching plant, or similarsystem. Where the solution to be tested does not act as an electrolyte,an electrolyte may be added to the sample to be analyzed prior to makingmeasurements.

[0073] While the system has been described with particular reference todetection of peroxyacetic acid, the system is also applicable todetection of hydrogen peroxide and other oxidizing species. By selectingan appropriate read voltage to maximize the contribution of the speciesto the overall output current and by choosing counter and workingelectrodes which are particularly suited to the species to be detected,the system may be tailored to the specific detection of a variety ofspecies.

[0074] The invention has been described with reference to the preferredembodiment obviously, modifications and alterations will occur to othersupon reading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

Having thus described the preferred embodiment, the invention is nowclaimed to be:
 1. A decontamination process comprising: (a) circulatinga decontaminant solution which includes peroxyacetic acid though atreatment vessel which contains items to be decontaminated; (b)withdrawing a sample of the decontaminant solution into a chamber tocontact a working electrode and a counter electrode; (c) pulsing avoltage between the working electrode and the counter electrode at aselected voltage relative to a reference electrode, the voltage beingselected such that an output current generated is correlated to aconcentration of the peroxyacetic acid in the sample and substantiallyindependent of a concentration of at least one other oxidizing speciesin the sample; and (d) measuring the current generated.
 2. The processof claim 1, wherein the other oxidizing species is hydrogen peroxide. 3.The process of claim 1, further including, prior to step (c): applying aconditioning pulse sequence between the working electrode and thecounter electrode, the conditioning pulse sequence including a negativevoltage pulse and a positive voltage pulse.
 4. The process of claim 1,further including prior to step (a): circulation a preconditioningsolution including buffers and wetting agents; with drawing a sample ofthe preconditioning solution into the chamber; and pulsing voltagesbetween the working and counter electrodes which electrochemicallyremove residues from the working electrode.
 5. The process of claim 1,further including, prior to step (c): adjusting the temperature of thesample to a preselected temperature.
 6. The process of claim 1, furtherincluding, prior to step (c): flushing the chamber with thedecontaminant solution.
 7. The process of claim 1, wherein step (c)includes: selecting the voltage in the diffusion limiting range.
 8. Theprocess of claim 1, wherein step (c) includes: pulsing a voltage ofabout −1.4 volts, relative to a silver/silver chloride referenceelectrode.
 9. The process of claim 1, wherein step (d) includes:measuring the current generated at about ten to fifteen seconds afterthe start of the voltage pulse.
 10. The process of claim 1, wherein stepc) includes: pulsing the voltage at a rate of about 25 pulses/second.11. The process of claim 1, further including, after step (d): signalingan indication of the current measured to a control system, which, in theevent that the current measured is below a predetermined minimum level,conducts at least one of the following steps: aborting thedecontamination process; extending the time of the decontaminationprocess to compensate for the peroxyacetic acid concentration;controlling the addition of additional peroxyacetic acid to thecirculating decontaminant solution; and providing a signal whichindicates that the peroxyacetic acid concentration is below thepredetermined minimum level.
 12. A method of detecting a first oxidizingspecies in a solution which also contains a second oxidizing species,the method comprising: disposing a working electrode and a counterelectrode in the solution to be tested; pulsing a read voltage in thediffusion limiting range across the working electrode and the counterelectrode; and detecting current flowing between the working electrodeand the counter electrode, the read voltage being selected such that thecurrent flowing is substantially dependent on a concentration of thefirst oxidizing species and substantially independent of a concentrationof the second oxidizing species in the solution.
 13. The method of claim12, further including: converting the detected current flow into anindication of the concentration of the first oxidizing species in thesolution.
 14. The method of claim 12, wherein the read voltage isselected such that a contribution of the first oxidizing species to thecurrent flowing is at least ten times that of an equivalentconcentration of the second oxidizing species.
 15. The method of claim12, further including: detecting a temperature of the solution adjacentthe electrodes; and correcting the detected current flowing for adifference between the detected temperature and a preselectedtemperature.
 16. The method of claim 12, further including: increasingthe first oxidizing species concentration in the solution when theconcentration is below a preselected minimum level.
 17. The method ofclaim 12, wherein the first oxidizing species includes peroxyacetic acidand the second oxidizing species includes hydrogen peroxide.
 18. Themethod of claim 17, wherein the peroxyacetic acid concentration is inthe range of 100 to 3000 ppm.
 19. The method of claim 17, wherein theperoxyacetic acid concentration is determined in under one minute. 20.The method of claim 17, wherein pulsing the read voltage, and detectingcurrent flowing are repeated at intervals of from about fifteen tothirty seconds.
 21. The method of claim 12, further including prior tothe pulsing step: applying alternating 1 to 10 seconds voltage pulses of−1.5 to −2.5 volts and +2.0 to +3.5 volts between the working andcounter electrodes.
 22. The method of claim 21, wherein the alternatingvoltage pulses are applied prior to the electrodes being disposed in thesolution to be tested while the electrodes are disposed in a solutionfree of the oxidizing species.
 23. The method of claim 22, wherein thealternating pulses are pulses +2.5 volts for 4 seconds and −2.0 voltsfor 2 seconds.
 24. The method of claim 11, further including: adding anelectrolyte to the sample of the solution to be tested.
 25. Adecontamination apparatus comprising: a decontamination vessel whichreceives items to be decontaminated; a fluid flow path which circulatesa decontaminant in a solution comprising a first oxidizing species tothe vessel; a sensor system fluidly connected with the fluid flow pathfor specifically detecting the decontaminant in the decontaminantsolution, the system comprising: a chamber which receives a sample ofthe decontaminant solution from the fluid flow path, a working electrodeand a counter electrode disposed within the chamber to contact thesample of decontaminant solution; and an amperometric controllerelectrically connected with the working and counter electrodes which:selectively pulses a preselected read voltage across the workingelectrode and the counter electrode, and detects an output currentflowing in a circuit including the working electrode, the counterelectrode and the solution, the read voltage being selected such thatthe output current is substantially dependent on a concentration of thefirst oxidizing species and substantially independent of a concentrationof at least a second oxidizing species in the solution.
 26. Theapparatus of claim 25, wherein the sensor system further includes: areference electrode disposed in the chamber, the amperometric controllermaintaining a potential between the working electrode and the referenceelectrode.
 27. The apparatus of claim 25, further including: a heaterfor heating the sample to a preselected temperature prior to pulsing theread voltage.
 28. The apparatus of claim 25, wherein the workingelectrode is formed from glassy carbon.
 29. The apparatus of claim 25,wherein the electrodes are laid down on a common substrate.
 30. Theapparatus of claim 25, further including a thermistor for detectingtemperatures in a portion of the solution adjacent the electrodes forcorrecting the measured current for temperature fluctuations.
 31. Theapparatus of claim 25, further including a computer which receivescurrent signals from the amperometric controller and determines aconcentration of the first oxidizing species.
 32. The apparatus of claim25, wherein the computer includes a look-up table which determines theconcentration of the first oxidizing species from the current output ata given temperature.
 33. The apparatus of claim 31, further including adispenser for dispensing additional amounts of the first oxidizingspecies into the solution, and wherein the computer directs thedispenser to dispense an additional amount of the first oxidizingspecies into the solution when a preselected minimum level of the firstoxidizing species in the solution is detected.
 34. The appartus of claim25, wherein the amperometric controller further applies alternatingpositive and negative voltage pulses across the working and counterelectrodes for electrochemically cleaning the working electrode.